A hydraulic drive system which executes a load sensing control so that the delivery pressure of the hydraulic pump becomes higher than the maximum load pressure of the actuators by a target differential pressure is called a load sensing system, which is described in Patent Literature 1, for example.
The hydraulic drive system described in the Patent Literature 1 comprises an engine, a hydraulic pump of a variable displacement type which is driven by the engine, a plurality of actuators which are driven by hydraulic fluid delivered from the hydraulic pump, a plurality of flow rate/direction control valves which control flow rates of the hydraulic fluid supplied from the hydraulic pump to the actuators, a detecting circuit which detects the maximum load pressure of the actuators, control means which executes the load sensing control so that the delivery pressure of the hydraulic pump becomes higher than the maximum load pressure of the actuators by target differential pressure, and an unload valve which is arranged in a pipeline connecting the hydraulic pump to the flow rate/direction control valves and restricts the increase in the delivery pressure of the hydraulic pump by shifting to an open state and returning the hydraulic fluid from the hydraulic pump to a tank when the delivery pressure of the hydraulic pump exceeds the sum total of the maximum load pressure and preset pressure.
A load sensing system equipped with an exhaust gas purification device has been described in Patent Literature 2. In this system, the exhaust gas purification device attached to the exhaust pipe is equipped with an exhaust resistance sensor. When the measurement by the exhaust resistance sensor has reached a prescribed level or higher, a control device of the load sensing system outputs signals to control the unload valve and a regulator of the main pump (hydraulic pump), by which the delivery flow rate and the delivery pressure of the hydraulic pump are raised at the same time and a certain hydraulic load is put on the engine. Due to the increase in the engine load, the output power of the engine increases, the exhaust gas temperature rises, the oxidation catalyst inside the exhaust gas purification device is activated, the deposits on the filter (filter deposits) are combusted, and the filter is regenerated.